Protect your plants from Powdery Mildew

Figure 1: PM on Tomato

 Powdery mildew is a fungal disease that affects a wide variety of plants. The affected plants have a dusted-with-flour appearance with white to gray powdery spots on the surface of leaves and fruits (Fig 1). It causes cosmetic damage, and defoliation and decreases the quantity and quality of fruit (Fig. 2).

Disease and Development 
There are many different species of (Powdery Mildew) fungi. Most species only attack specific plants in the same or closely related plant families.

Figure 3: PM on roses

For example, powdery mildew on roses does not spread to grapes.

Powdery Mildew fungi overwinter in buds, twigs, bark, and plant debris. Their spores are carried by wind to new hosts. Low relative humidity during the day favors spore dispersal.

Fig 2: PM on grapes

The optimum conditions for spores to germinate are temperatures between 68 to 77°F and high relative humidity (> 70%). Powdery Mildew spores can germinate in the absence of water and can infect in arid conditions (40% humidity). Water on plant surfaces can kill or inhibit germination. Spores and fungal growth are sensitive to extreme heat (above 90°F) and direct sunlight. Shady conditions are favorable for PM development.

Management
The disease triangle is a useful tool to understand and manage diseases like Powdery Mildew (Fig. 3). The severity of Powdery Mildew infections is a function of the presence of the pathogen, the host susceptibility, and the environment. Powdery Mildew can be managed by good cultural practices.   

Fig 3: The Disease Triangle

The best control is to plant only resistant species or cultivars. Avoid excess fertilizer since plants with abundant succulent tissues are more susceptible to infection. The fungi can be eliminated by removing and burning all infected plant tissues and debris. Tools must be disinfected after use.

Another good control method is making environmental conditions less favorable for the disease. Avoid dense canopies by proper plant spacing and pruning to provide good air circulation, reduce humidity and increase sunlight penetration. Plants should not be planted in poorly drained soils or shady damp areas.

Overhead watering of the leaves during the day and when humidity is low can reduce infection. Water washes spores off the plant and inhibits its germination. This control method is not recommended for vegetables since it may contribute to other pest problems.

Powdery Mildew seldom needs chemical control in residential plantings, but it may be warranted in commercial plantings of susceptible crops like grapes and cucurbits. Chemicals are most effective when combined with cultural control. They should be applied before the disease symptoms appear. Further spread may be stopped or slowed with chemical treatment. Materials like horticultural oil or neem oils have a low environmental impact. Apply every 7 to 14 days throughout the season. Do not apply to drought-stress plants when the air temperature is above 85° nor within 2 weeks of a sulfur spray. Wettable sulfur is very effective in preventing infection. Like oil application, it should not be applied when the air temperature is over 90° nor within 2 weeks of an oil application.

Powdery Mildew

Does something not look right on your plants or in your lawn and garden? 

What if I told you that it could possibly be a fungus that is infecting them all?

 Powdery Mildew could be the suspect we are looking for. Powdery mildew looks like white to gray spots with a talcum powder like growth. The mildew is host specific, i.e., the mildew that can be affecting your lilac and elm trees would not affect your grass, or mildew on grapes would not affect your lilac. Mildews thrive in warm dry climates. When plants have high humidity, poor air circulation, or are located in shaded areas, it allows the mildew to have an ideal growing environment.

What can we do to fix it?

The simplest and easiest option is to buy plant/seed varieties that are resistant to powdery mildew, i.e., roses, cucumber, squash, and beans.

 Not everything we want to plant will have an available resistant variety; therefore powdery mildew needs to be managed culturally and chemically.

Cultural practices:

·         Prune with good air circulation in mind.

·         Do not over fertilize and possibly look into slow release fertilizers for future applications.

·         Do not overhead water to help reduce humidity and reduce chances of spread of other fungus problems.

Chemical practices:

●   Fungicides- function as protectants, eradicants or both.  A protectant fungicide is a preventative to protect against new infections from occurring. An eradicant fungicide kills an actively growing infection. For highly susceptible plants, apply protectant fungicide before disease appears. Eradicants should be applied at earliest appearance of disease.

 ●    Horticultural oils- can be an effective eradicant in plants with mild to moderate symptoms and has some protectant effects with products such as Saf-T-Side Spray Oil, Sunspray Ultra-Fine Spray Oil, neem oil or jojoba oil.

●    Sulfur- usually a spray that is only effective before the symptoms have appeared. Copper may also be used in the same manner. It has a lower effective rate.
●    Bacterial Fungicides- made of a pathogen that kills the fungus and helps prevent future infections. These are also safe for beneficial insects, pets, and people. 

 With applying any chemicals, you want to make sure that you are reading the labels and ensuring that you have correct target pest on plants, proper protective equipment, and applying at correct rates. The label is law and we do not want to damage the environment with an application applied incorrectly. Now with our new knowledge on how powdery mildew works, we can better manage it in our gardens going forward.

 Consider signing up for land and garden pest alerts at the Pest Alert Network in order to find out about pests as soon as they are found in our area. These are the alerts for Powdery Mildew.